Hm, PC Gamer blames Valve an Team Fortress 2 for starting this in 2007.
And I guess that's probably true, though the content from TF2 was just new weapons, hats and maps. The game _systems_ generally weren't changing.
Whereas Minecraft was adding entire new realms (The End), creatures, systems (enchanting, potions, etc). Because it was an unfinished, early-access game, it taught people that you pay for a game and the game just keeps expanding and expanding.